If you're not sure wether to feel insulted, then it's best to err on the side of not. It might be a physical thing, or it might be that you've never given any indication that you have a girlfriend. The average teenage girl is something of a gossip whore, I don't know this person, but unless she's in the minority that doesn't care about gossip, then the revelation that a relationship was occuring right under her nose probably outwieghts being the only one who can look forward to a dry spell.